Scope dark mode change only to the presenting sheet (#6144)

* Scope dark mode change only to the presenting sheet, instead of the whole window

* Apply review suggestions to the presentation colour scheme modifier

API:
- Take a `ColorScheme?` at the SwiftUI level instead of a `UIUserInterfaceStyle`
  and map it once inside the representable (`nil` means no override).
- Rename to `presentationColorScheme` and the file to match.
- Restore the `preferredColorScheme` property on the media preview details and
  redact confirmation views, and the `nil` at the media timeline call site, so
  no UIKit types leak into the views.

Cleanup:
- Walk the responder chain with `sequence(first:next:)` instead of a while loop.

* Re-record the media upload preview iPad snapshots

The Cancel button no longer has an explicit tint, so its label colour changed
very slightly from `textActionPrimary` to the default. Only the iPad references
include the toolbar, which is why the iPhone ones were unaffected.

PreviewTests/Sources/__Snapshots__/PreviewTests/mediaUploadPreviewScreen.iPad-en-GB-0.png

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+size 86285

PreviewTests/Sources/__Snapshots__/PreviewTests/mediaUploadPreviewScreen.iPad-pseudo-0.png

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